It's all about the work. At least that's true for industrial robots, utility robots, and one day, humanoids. To accomplish this work, laboring bots are equipped with all manner of manipulators outfitted with grippers and tools (called end effectors in botspeak). Grippers run the gamut from simple two- or three-fingered claws to more sophisticated end effectors with human-like fingers. Other tools include just about anything a human might wield (drills, cutters, wrenches, welders, and so forth).
In mobile robots, manipulators and end effectors aren't very common. Surf the numerous robot sites on the Web, and you'll see tons of cool mobile robots, but few of them will have arms or working tools. If Barbie is fond of complaining that "math is hard," robot-builder Malibu Stacey will tell you that robot manipulators and end effectors are even harder.
In many ways, it's not the level of difficulty that's the problem, it's integrating the arms/effectors into a mobile robot design. Like a lot of amateur bot builders, I have several robotic arms on my bench, and a bunch of mobile robots, but I've never made a mobile robot with an arm. As with a walking drive train, as soon as one starts to think about a manipulator, there are greatly increased power needs (more batteries/more weight), numerous servo motors to contend with (for all of the arm's joints), more complex computer programming to consider, and a reliable gripper to engineer and build. Also, an arm dangling out into the world can be a danger to a robot when it bangs into something.
One other issue that builders of end effectors have to contend with is the amount of force that the gripper, fingers, or other tool will deliver.
It's a scary proposition to shake a robot's hand when you're not quite sure whether it's going to know when to stop squeezing. One way this is easily done is to use servo motors that have mechanical stops built into them that only allow the shaft of the motor to rotate a fixed number of degrees. Another more complicated method involves pressure sensors in the end effector that give the robot feedback (called force feedback) about the presence of an object in its grasp. Personally, until more of the bugs are worked out, I don't make it a practice of shaking too many robot hands.
You interact with your world through your five senses: sight, sound, smell, taste, and touch. Robots can have all of these senses (okay, I don't know of any bots that can actually taste), and dozens more. A sensor is any device on a robot that receives input from the outside world and passes that input on to the robot's control system. Let's look at a few of the most common types of robot sensors.
A pressure sensor is basically a switch that, when turned on (or off), sends a signal to the control circuitry of a robot to do something (usually back up or otherwise move to a new spot). Probably the most common type is the bump sensor, often a fender or skirt on the robot that, when hit, presses a switch to which the controller responds. Another common type is the feeler or whisker (common on robo-critters). This is simply a wire that, when pressed or bent, engages a switch. Pressure sensors are probably the most common type of sensors around, and robot builders are constantly experimenting with more effective ways of building them.
There are a number of ways of using light sensors, but the basic idea is to use light waves sent out and received by special photosensitive components to control a robot's actions. In one frequently used navigation scheme, this is accomplished by putting an infrared (IR) transmitter on the bot that sends an invisible light beam out into the environment. That light is reflected off of objects back to a special infrared receiver elsewhere on the bot. The angle of that reflected light changes depending on the proximity of the robot to the object that's reflecting the light. The robot can use this change in angle to measure the distance and trigger an appropriate action (such as an obstacle avoidance sequence).
Light sensors are also used in line-following navigation. Commonly, a black or white line is put down to create a "track" for the robot to follow. Photosensitive sensors, on the front or bottom of the bot, straddle the line. If one of the sensors registers a high degree of change in light intensity (by crossing the line), it triggers the motors to adjust course to keep the line between the two sensors (and the robot on track).
Another use of light sensing is to engineer circuits outfitted with light sensors that make the robot "attracted" to light (in other words, it moves in the direction of the most intense light source), or repulsed by the light (it moves away). These "behaviors" are called, respectively, photovoric (photo meaning light, vore being Latin for "swallow up") and photophobic.
The most common type of sound sensor makes use of sonar technology. Sonar sensors basically use the speed of sound to measure distance and use that information to aid in robotic navigation. Inaudible sound waves (outside the range of human hearing) are projected out from a transmitter on the robot, bounce off of surfaces, and return to a receiver on the robot. The time it takes for the sound waves to return to the sonar sensor (also called an ultrasonic range sensor) is used to calculate the robot's distance from an approaching object. This data is then passed along to the control circuitry of the robot and appropriate action is taken.
Another type of sound sensing involves the use of microphones. A robot outfitted with mics can be programmed to move toward a sound source, move a certain body part, or move all of its body parts. Entertainment robots such as Sony's forthcoming SDR-4X humanoid take in sounds via microphones so that the robot can dance (the sound input activates the robot's many servo motors). Robots are also being programmed to respond to certain voice characteristics (such as a raised voice or lowered voice). Of course, microphones are also used on robots that are programmed to respond to sets of human voice commands.